(1) Field of the Invention
The present invention relates to a variable optical delay circuit, for example, a variable optical delay circuit available as a variable differential group delay (DGD: Differential Group Delay) compensator designed to compensate for a delay between two polarization modes perpendicularly intersecting each other for the polarization mode dispersion compensation in an optical communication system.
(2) Description of Related Art
For the recent large-capacity transmission, there has been taken up the problems of polarization mode dispersion (PMD: Polarization Mode Dispersion). In particular, it has been said that the PMD compensation is essential in a 40-Gbps (Giga bits per second) system.
Although a linearly-polarized optical signal pulse train ideally incident on an axisymmetric optical fiber is detectable in a state of linearly polarization even after a long-distance transmission, in fact a core distortion of an optical fiber occurs due to difference in manufacturing of optical fibers or variations of installation environments with time. This principally causes a split of a linearly polarization into two polarization components [a polarization component (horizontal polarization) whose vibrational vector is parallel with a plane of incidence and a polarization component (vertical polarization) perpendicular thereto] which perpendicularly intersect each other at an exit of the optical fiber, and a difference in speed between the polarization components develops, thus making it difficult to accomplish the normal detection of an optical signal pulse.
In general, the PMD compensation is carried out through the use of a polarization controller and an optical delay circuit. That is, a polarization controller is made to adjust the polarization direction of an optical signal while a variable optical delay circuit has a function to cancel the speed difference between two polarization components developing during the optical fiber transmission as mentioned above.
As a concrete example, FIG. 10 shows a configuration of a conventional variable optical delay circuit proposed in Japanese Patent Laid-Open No. HEI 6-67219. The variable optical delay circuit shown in FIG. 10 has a basic structure in which sets of polarization plane control devices 300-1 to 300-N (N represents an integer equal to or more than 2) and polarization beam splitters (PBS: Polarization Beam Splitter) 400-1 to 400-N and 500-1 to 500-N are alternately disposed at a plurality of stages, and one (for example, horizontal polarization) of an incident beam 101 having two polarization components which perpendicularly intersect each other is reflected through a polarization beam splitter 100 toward a ½-waveplate 200 side and the polarization plane control elements 3-i (i=1 to N) selectively determines a set of the polarization beam splitters 400-i and 500-i (in FIG. 10, a set comprising the polarization beam splitters 400-2 and 500-2) to be used for reflexing the incident beam 101 (horizontal polarization), thus varying the optical path length of the incident beam 101 according to polarization components perpendicularly intersect each other for providing an appropriate delay.
Therefore, in this case, the optical path length varies in a stepwise (digital-like) fashion and, hence, the compensation quantity varies in a digital-like fashion. Incidentally, the aforesaid ½-waveplate 200 has a function to make a conversion of horizontal polarization→vertical polarization with respect to the incident beam 101 inputted from the polarization beam splitter 100 and further to make a conversion of vertical polarization→horizontal polarization with respect to the incident beam 101 inputted from the polarization plane control device 300-1. The incident beam 101 converted into the horizontal polarization is reflected by a polarization beam splitter 900 to be outputted in the same direction as the direction of the incidence on the polarization beam splitter 101.
In addition, as a conventional variable optical delay circuit, for example, there has been known a circuit shown in FIG. 11. The variable optical delay circuit shown in FIG. 11 is proposed in the document “F. Heismann, “Polarization mode dispersion: fundamentals and impact on optical communication systems.”, Proc. ECOC '98, Tutorials, pp. 51–79.” wherein a polarization beam splitter 600 splits an incident beam into two polarization components (horizontal and vertical polarizations), with one (for example, horizontal polarization) being outputted intact to a polarization multiplexer 700 while the other (vertical polarization) being outputted to a reflecting mirror 801.
Moreover, the vertical polarization beam reflected on the reflecting mirror 801 is successively reflected in the order of a reflecting mirror 802, a reflexing mirror 803 and a reflecting mirror 804 to be incident on the polarization multiplexer 700 after passing through an optical path different from that of the one horizontal polarization beam, where it is coupled with the horizontal polarization beam. In this case, the reflexing mirror 803 is of a movable type whereby the relative distance therefrom to the reflecting mirror 802 is mechanically adjustable (movable).
Therefore, the optical path length of the vertical polarization beam varies in accordance with the movement quantity of the reflexing mirror 803 and, hence, the delay quantity (compensation quantity) corresponding thereto is attainable.
However, in the case of the conventional variable optical delay circuit mentioned above with reference to FIG. 10, since the compensation quantity varies in a digital-like fashion, difficulty is experienced in compensating continuously for arbitrary DGD. In addition, a characteristic degradation occurs due to variation in refractive index of optical parts such as the polarization beam splitters 100, 400-i and 500-i which stems from variation in ambient temperature.
Furthermore, in the conventional variation optical delay circuit mentioned above with reference to FIG. 11, the mechanical movement of the reflexing mirror 803 leads to a lowered response speed and a lowered stability and further to an increase in apparatus scale. Moreover, not only a difference in loss appears in each optical path, i.e., between the horizontal polarization and the vertical polarization, but also the loss difference varies depending upon the delay quantity.
Incidentally, among other conventional techniques, there are, for example, (1) a variable optical attenuator proposed in Japanese Patent Laid-Open No. HEI 9-288256, (2) an optical isolator proposed in Japanese Patent Laid-Open No. HEI 6-250121, (3) an optical circulator proposed in Japanese Patent Laid-Open No. HEI 5-34633, (4) an optical isolator proposed in Japanese Patent Laid-Open No. HEI 5-313094, and others.
Each of these techniques takes advantage of the characteristic of a doubly refracting crystal which can spatially separate polarization components perpendicularly crossing each other. That is, for example, as illustratively shown in FIG. 12, when an anisotropic axis (equally referred to as an optical axis or crystal axis) of a doubly refracting crystal is disposed so as to make an angle of approximately 45 degrees with respect to a traveling direction of inputted light, the inputted light is spatially separated into an ordinary light component (for example, a component in a direction perpendicular to the paper surface) 111 and an extraordinary light component (for example, a component parallel to the paper surface) 112. It takes advantages of this characteristic.
Concretely, for example, this is written in the paragraphs 0017 to 0021 and FIGS. 2 and 3 in the aforesaid (1), the paragraph 0018 in the aforesaid (2), the paragraphs 0011 to 0013 and FIG. 3 in the aforesaid (3), and the paragraph 0012 and FIGS. 1 and 2 in the aforesaid (4), and others.
However, because of spatially separating the polarization components perpendicularly intersecting each other, these techniques cannot produce a function as a variable delay circuit, although they can provide a variable optical attenuator, an optical isolator and an optical circulator. Moreover, the difference in optical path between the polarization components causes a loss difference.